VPSA processes and systems are known in the art for separating components of a feed gas mixture. Such a gas mixture contains a more readily adsorbable component (i.e., a "more preferred" gas) and a less readily adsorbable component (i.e., a "less preferred" gas), and is passed through an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at an upper adsorption pressure. The bed is thereafter depressurized to a lower desorption pressure (e.g. a vacuum) for desorption of the more readily adsorbable component and its removal from the bed prior to introduction of additional quantities of the feed-gas mixture. In a multiple bed VPSA system, the beds are cyclically operated through the same series of process steps, but the step sequence in one bed is offset from the same step sequence applied to another bed. The step sequence offset is accomplished to allow use of common feed and exhaust systems and to achieve process and energy savings.
In conventional VPSA systems, multiple adsorber beds are commonly employed, with each bed subjected to a VPSA processing sequence on a cyclic basis so as to enable efficiencies to be achieved. VPSA systems are often used to separate oxygen from an input air stream. At certain times during operation of a VPSA system, either a feed blower or a vacuum blower, or both, are caused to operate in an "idle" mode, where they do not interact with associated adsorbent beds to actively move feed or exhaust gas through the system. Such operation is hereafter referred to as the unload state. The term turndown state will hereafter be used and will refer to the condition when: both the feed and vacuum blower are set into the unload state (idling) for an extended period of time; and the VPSA system is not producing product.
During the VPSA process, gas streams are frequently expanded when pressure transferred during the process. Such gas transfer takes place at both the product and feed ends of the adsorber bed. Energy recovery from expanding gas streams in VPSA processes has long been a goal in systems design.
Most present VPSA systems and, in particular, two-bed systems incorporate process steps which throttle gas streams for the purpose of pressure transferring the gas. The throttling results in lost power and added inefficiencies. Energy recovery in the prior art has also employed a natural aspiration of the feed air during vacuum conditions, at the beginning of the VPSA cycle. The natural aspiration method requires an additional air inlet regulation system and results in only a modest reduction in the feed gas compression requirement. Nor does the aspiration system recover energy from the expanding stream, but rather merely provides an air inlet without additional power consumption.
Other prior art teachings related to energy recovery in gas separation systems are as follows. U.S. Pat. No. 5,429,666 to Agrawal et al. describes a vacuum swing adsorption (VSA) process which employs two beds that operate with product pressurization and pressure equalization between the beds. Simultaneous operation of the process steps, for the purpose of continuous utilization of feed and vacuum blowers, is described. The Agrawal et al. process employs a natural aspiration of feed air as an energy recovery process. The system attempts to lower feed power by utilizing the low adsorber bed pressure at the beginning of a cycle to allow for some fraction of the feed air to be drawn directly into the bed, without need for an air compressor. Such an ambient feed does nothing to recover energy that is available from the expansion of the feed air.
U. S. Pat. No. 4,995,889 to Abel et al. describes a method for regulating product flow of an adsorption air separation system, especially under conditions of discontinuous product flow that result from variable customer demand. A control valve is connected to the product line of the separation apparatus and controls flow of the product through a variable or fixed orifice device that is upstream of the control valve. A differential pressure controller, which senses pressure upstream and downstream of the orifice device, is used to operate the control valve.
U.S. Pat. No. 5,096,469 to Keefer details an adsorption air separation process which utilizes oscillations of a liquid column to change the volume of variable displacement chambers in order to create cyclic pressure changes that are required for the pressure swing process. In effect, the inertia of the oscillating fluid provides an energy exchange between air separation chambers.
U.S. Pat. No. 5,183,483 to Servido et al. describes a pneumatic control process for a pressure swing adsorption (PSA) process. Adsorption, desorption and equalization phases are connected through use of two 3-way valves and a single compressor. By controlling the operation of the 3-way valves, the compressor can be used for adsorption and desorption or can be allowed to operate unloaded as well.
U.S. Pat. No. 5,518,526 to Baksh et al. describes a PSA process which overlaps various steps to reduce total cycle time and to achieve improved efficiency and productivity. A unique step is described as being the simultaneous evacuation of a bed undergoing an equalization rising step, while the other bed is undergoing an equalization falling step. The next step in the cycle is simultaneous product and feed pressurization at opposite ends of the bed, followed by feed pressurization to the desired adsorption pressure.
U.S. Pat. No. 5,042,994 to Smolarek (Applicant herein) describes a method for controlling a PSA system by the monitoring of a variable volume storage vessel during nitrogen production applications. The process cycle contains two steps where the feed blower and vacuum blower are idle. The first step is a counter-current oxygen repressurization step of a previously desorbed bed, while an adsorbed bed undergoes a blow-down of product nitrogen. The second step when the process machines are idled and not utilized occurs during a turndown step when the level of the variable volume product storage vessel is monitored in order to determine variations in customer demand. Thus, Smolarek teaches that the measure of idle time is proportional to some measure of customer demand. Smolarek further mentions that power reduction and energy savings can be achieved under turndown conditions by idling the machines proportionally with customer demand, while maintaining product purity.
Notwithstanding the substantial development efforts that have been directed at improvements of pressure swing adsorption (PSA) and VPSA processes and systems, there is a continuing need for efficiency improvements therein.
Accordingly, it is an object of this invention to provide a pressure swing adsorption system which exhibits energy usage efficiencies.